Electromagnetic testing emt chapter 18 aerospace applications

Electromagnetic Testing

MFLT/ ECT/ Microwave/RFT Chapter 18 Aerospace Applications of Eddy Current Testing 25th March 2015 My ASNT Level III Pre-Exam Preparatory Self Study Notes

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Aerospace Applications

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http://www.wired.com/2014/06/why-harrier-jets-crash/

Aerospace Applications

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http://www.wired.com/2014/06/why-harrier-jets-crash/

Aerospace Applications

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http://fr.yoyowall.com/wallpaper/militaire-osprey.html

Aerospace Applications

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Aerospace Applications

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Aerospace Applications

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Aerospace Applications

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http://chinesemilitaryreview.blogspot.com/2011/10/peoples-liberation-army-air-forces-j-10.html

Aerospace Applications

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http://chinesemilitaryreview.blogspot.com/2011/10/peoples-liberation-army-air-forces-j-10.html http://fr.yoyowall.com/wallpaper/militaire-osprey.html

Aerospace AerospaceApplications Applications

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http://chinesemilitaryreview.blogspot.com/2011/10/peoples-liberation-army-air-forces-j-10.html http://fr.yoyowall.com/wallpaper/militaire-osprey.html

Aerospace AerospaceApplications Applications

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Aerospace Applications

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Aerospace Applications

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Aerospace Applications

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Vertical Take Off Plane

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https://www.youtube.com/embed/4qPWguMKGiI

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Vertical Take Off Plane

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https://www.youtube.com/watch?v=7YwlYdiV23M

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Vertical Take Off Plane

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https://www.youtube.com/watch?v=pz6sc0bgWcM

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Vertical Take Off Plane

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https://www.youtube.com/embed/BLiLK0KI_JQ

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Charlie Chong/ Fion Zhang

Fion Zhang at Shangahi 25th March 2015

Shanghai 上海 Charlie Chong/ Fion Zhang

http://meilishouxihu.blog.163.com/

Greek letter

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Chapter Eighteen: Aerospace Applications of Eddy Current Testing

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18.1 PART 1. Introduction to Eddy Current Testing of Aircraft 18.1.1 Typical Applications Applications of eddy current tests in the aerospace industry include the following: (1) measurement of effects that alloying elements, heat treatment, test object dimensions, cladding, liftoff and temperature have on eddy current conductivity values; (2) sorting of mixed aluminum alloys; (3) evaluation of overaging or heat damage to aluminum; (4) detection of alpha case hardening on titanium alloys and detection of titanium aluminide in aluminum brazed titanium honeycomb; (5) eddy current testing of bolt holes in aircraft structures; (6) analysis and applications using impedance plane techniques to measure metal thinning and metal spacing and to detect exfoliation corrosion and surface cracks in aluminum aircraft structures; and (7) low frequency testing of aircraft structures for the detection of subsurface cracks.

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Aluminum Exfoliation

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Aluminum Exfoliation

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Aluminum Exfoliation

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Titanium - Brazing and Soldering Where welding is neither practical nor economically feasible, it may be desirable to consider the brazing or soldering of titanium. Brazing is a joining process wherein the filler metal has a melting point greater than 800째F (430째C), but less than that of the materials to be joined.

Soldering, on the other hand, employs filler metal which melts at less than 800째F (430째C) and is commonly applied to thin-gauge material or wire.

Of the conventional metals employed in the brazing of steel, only pure silver and aluminum have been applicable to titanium with satisfactory results. High strength aluminum alloys have produced brittle impractical joints. Other metals such as zinc and tin will not adequately wet the titanium surface.

Gas Brazing. Of the many methods for gas brazing other metals, only the oxyacetylene gas has been found effective to date with titanium in that the gas employed does not embrittle the material. With a similar brazing method employing pure aluminum in place of pure silver, reasonably ductile but low strength joints are obtained.

In gas brazing aluminum to titanium, the titanium part is dipped into molten aluminum; the aluminum-clad titanium is then brazed to the aluminum part by conventional aluminum brazing techniques.

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http://www.keytometals.com/Article136.htm

Brazing of Ti & TiAl- Al-12Si-0.8Fe-0.3Cu / Ti-37.5Zr-15Cu-10Ni

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http://titanium-brazing.com/products.html

Titanium Aluminide - Through extensive research beginning in the mid 1990s, AMG developed titanium aluminides (TiAl). Initial applications for this high strength, low weight material included components for Formula 1 race cars and have evolved into an alloy for aerospace applications. Titanium aluminide alloys offer superior high temperature performance with low weight for turbine blades and are as strong as nickel based alloys, but at only half the weight.

These alloys will allow for smaller aerospace engines, reduced fuel consumption and less hazardous emissions, thus reducing the impact on the environment.

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http://www.amg-nv.com/Innovation/Titanium-Aluminide/default.aspx

Titanium Aluminide Alloy - TiAl alloy is a new alloy based on intermetallic compounds and is characterized by the properties of light weight and high strength. Two-phase TiAl alloys have been receiving considerable attentions because of their attractive properties, such as the low density, excellent high-temperature strength, and good oxidation resistance.

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http://www.keytometals.com/page.aspx?ID=CheckArticle&site=ktn&NM=269

Titanium Aluminide

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Titanium Aluminide

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http://www.docin.com/p-43452610.html

Titanium Aluminide

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https://www.is.mpg.de/15354255/07_Industrial_Materials

Titanium Aluminide

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18.1.2 Testing of Aluminum Alloys The widespread use of aluminum alloys in critical aerospace structures requires careful control during manufacture, to ensure: 1. that each test object is made from the specified aluminum alloy and 2. that the properties resulting from the composition and heat treatment of these alloys are those required for the specific application.

The electrical conductivity of aluminum alloys depends on their alloy content and the effects of prior processing. Thus, eddy current conductivity tests provide unique versatility in their capabilities for detecting mixed alloys or alloys having properties changed by thermal processing. Eddy current tests can supplement or in some cases replace liquid penetrant tests for detection of surface connected discontinuities and can also be used to measure the presence of coating or cladding on substrates.

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In addition, eddy current tests are often used to test aerospace structures during maintenance overhauls and to detect damage incurred in service.

Such tests can be used to detect fatigue cracks (including those near fasteners) resulting from cyclic loading during flight, takeoff or landing. They can also be used to indicate the extent of corrosion damage or to identify portions of aircraft structures damaged by fire. In this way, it is possible to identify damaged components that need to be removed and replaced to restore the craft to airworthiness.

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Maintenance overhauls

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18.2 PART 2. Eddy Current Tests of Metal and Coating Thickness 18.2.1 Coating Thickness on Metals In the aerospace industry, numerous metal parts are coated or plated to obtain special surface properties such as corrosion resistance, wear resistance or improved appearance. These coatings are prepared by various means, including electro-deposition, hot dipping, cladding and spraying. For controlling the thickness of the deposit and also for testing purposes, reliable and rapid measurements of coatings are necessary. There are many methods available, including magnetic, mechanical, optical, radiographic, ultrasonic and electromagnetic testing. Each of these methods has applications and more than one method may be used for the same application.

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â– Classification of Coatings and Base Materials Four general types of coatings and base materials lend themselves to eddy current testing. The operating procedure used with the test instrument is determined by the specific combination of materials. These material combinations are classified as follows: (1) metal foil and sheet or metal coating on a nonconductive base material, such as metallic film on glass, ceramics or plastics; (2) metal cladding with a higher conductivity than the base metal, such as copper, zinc or cadmium on steel or pure aluminum on aluminum alloy; (3) metal cladding with a lower conductivity than the base metal, such as nickel on aluminum; and (4) nonconductive coatings on a metallic base material, such as anodic film or paint on aluminum or such as other organic coatings on metals.

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■ Reference Standards for Thickness For eddy current thickness tests, at least three specimens with known metal thicknesses are needed as reference standards to calibrate the equipment.  One reference standard represents the minimum acceptable thickness,  the second represents the maximum acceptable thickness and  the third is from the middle of the range.

In some cases, a bare metal specimen is necessary. All reference standards must have the same conductivity, permeability, substrate thickness and geometry as the test objects.

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18.2.2 Metal Thickness Tests for measuring metal thickness are generally used on chemically milled sheet stock, thin walled tubing, metal foil bonded to nonmetallic materials and any parts that may experience thinning from corrosion. â– Null Balancing Phase sensitive equipment permits plotting metal thickness profiles on the impedance plane.

In Fig. 1, the operating point has a value of inductive reactance X = 800 and resistance R = 900. This point was purposely chosen so that the voltage vectors are about 45 degrees from the locus for change in thickness and 90 degrees, or normal, to the liftoff curve. When this is done, thickness changes also result in maximum instrument response. This happens because the vector to the thin metal is shorter than the vector to the thick metal, producing a large change between the two. However, the vectors to the liftoff curve are equal, resulting in no change in instrument amplitude response.

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FIGURE 1. Impedance plane thickness plot for Unified Numbering System A97075 wrought aluminum alloy, temper 6, at 10 kHz.

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A technique called null balancing can be used with any phase sensitive metered instrument. The operating point should be chosen so that the signal to be suppressed is normal to a line connecting it with the operating point; the variable to be measured is parallel or at least less than 45 degrees to the point.

Note that in Fig. 1 the short vector registers as low values on the meter and the long vector registers as large values. Null balancing is very useful for eliminating or reducing unwanted effects. Increasing or decreasing the signal gain control, to bring the meter reading back on scale with the signal level control, permits larger or smaller spreads of thickness to be displayed on the panel meter. Null balancing is discussed below, in the section on impedance plane analysis.

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18.2.3 Conductive Coating Thickness Cladding thickness measurement is used in two types of situations where the conductivities of two metal layers are very different.

1. The first situation, in which the cladding is a better conductor than the base material, pertains to copper, zinc or cadmium coatings on steel base materials.

2. The second situation, in which the cladding is the poorer conductor of the two metals, pertains to lead coatings on copper or to nickel coatings on aluminum.

The conducting coating thickness may be experimentally determined for a particular combination of coating metal, eddy current probe and test frequency by increasing the cladding thickness in steps until a given increase in thickness does not change the indicated meter reading (Fig. 2).

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FIGURE 2. Relative instrument readings for various thicknesses of copper plate on brass base.

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The layered examples in Fig. 3 result from matching arbitrary combinations of metals to illustrate how these combinations behave on the impedance plane.

The principles illustrated have practical applications. The impedance point of the thick copper base material would be a point on the conductivity locus as shown in Fig. 3a. If the copper base material is plated with aluminum, the probe coil impedance moves upward and to the left (clockwise) on the conductivity locus. The impedance on the conductivity locus approaches 3δ, corresponding to thick aluminum. If copper of various thicknesses is added on top of the aluminum, the probe coil impedance follows a locus downward and to the right of the conductivity locus. The movement of the impedance point (with increasing thickness) is not proportional to the thickness; the deflections for equal thickness changes become less as the thickness increases. The impedance point corresponding to a thickness about equal to one standard depth of penetration δ is shown in Fig. 3b. The impedance point finally converges rapidly on the copper conductivity locus at 3δ.

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The important fact illustrated in Figs. 3a and 3b is that the curves spiral with increasing curvature in a clockwise direction. The start of the spiral in each case approximates a straight line but as the thickness increases the spiral becomes more accentuated. The degree of spiraling and the shape of the curve will vary as the operating frequency is changed. A further example is the case of nonmagnetic but conductive coating on a magnetic substrate or base material, as illustrated in Fig. 3c for aluminum on steel. In this example, the inductive reactance decreases in a nonlinear fashion. The locus of points is to the right of the liftoff locus line. The locus crosses from the magnetic domain to the nonmagnetic domain on the impedance plane at rather thin values of aluminum. For plating on steel alloys, similar results would be obtained for cadmium at 14.5 MS·m–1 (25 percent of the International Annealed Copper Standard) and for chromium at 4.6 MS·m–1 (8 percent of the International Annealed Copper Standard).

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FIGURE 3. Impedance graphs showing coating thickness effects: (a) low conductivity on high conductivity material; (b) high conductivity coating on low conductivity material; (c) aluminum coating on steel at 25 kHz, in units of 25Îźm (0.001 in.).

Aluminum on Copper

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FIGURE 3. Impedance graphs showing coating thickness effects: (a) low conductivity on high conductivity material; (b) high conductivity coating on low conductivity material; (c) aluminum coating on steel at 25 kHz, in units of 25Îźm (0.001 in.).

Copper on Aluminum

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FIGURE 3. Impedance graphs showing coating thickness effects: (a) low conductivity on high conductivity material; (b) high conductivity coating on low conductivity material; (c) aluminum coating on steel at 25 kHz, in units of 25Îźm (0.001 in.).

Aluminum on Steel

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18.2.4 Nonconductive Coating Thickness The measurement of nonconductive coating thicknesses on metallic base materials is basically a liftoff measurement. The measurement is applicable to nonconductive coatings such as anodic coatings, paint or plastic on a conductive base. â– Normally, magnetic attraction gages are used to measure nonmagnetic coatings (conductive or nonconductive) on steel and â– an eddy current test is used to measure nonconductive coatings on nonmagnetic base materials.

The spacing from probe to test object is referred to as liftoff.

Figure 4 shows the impedance plane response that occurs when liftoff is increased at 100 kHz. The upper portion of the impedance plane is the magnetic domain where responses occur from ferromagnetic materials. The lower portion is the domain where responses are obtained from an aluminum alloy tested. Note the nonlinear changes along the liftoff locus for equal increments of spacing.

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FIGURE 4. Impedance graph showing liftoff effect at 100 kHz. Liftoff is spacing of probe to part in units of 25Îźm (0.001 in.).

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Vertical Liftoff

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18.2.5 Feasibility Criterion for Coating Thickness Tests For the coating material and the base material, the product of conductivity and permeability ((σ1 μ1) for one of the materials should be at least 1.5 times the product of conductivity and permeability (σ2 μ2) for the other material. Nonconductors such as air, organics and paint have a conductivity value of zero. Nonferromagnetic materials have a relative magnetic permeability equal to 1.0.

When measuring coatings on a metallic base, the total thickness of the base material plus the coating (if conductive) should be at least three standard depths of eddy current penetration, that is, should be ≥3δ. For eddy current tests of materials with various permeability and conductivity product values, standard depths of penetration as a function of frequency can be predicted by using Fig. 5.

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FIGURE 5. Chart for determining standard depth of penetration from relative magnetic permeability Îźr (henry per meter) and conductivity Ďƒ (siemens per meter) as percentage of International Annealed Copper Standard (%IACS).

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Depth of Penetration & Current Density

Where: μ = μ0 x μr Charlie Chong/ Fion Zhang

http://www.suragus.com/en/company/eddy-current-testing-technology

18.2.6 Metal Spacing There are times when a gap separates two metal sheets. The gap may be filled with a nonconductive adhesive layer or a nonmetallic shim or may be purposely produced as a fixed dimension. If it is desired to measure the gap or spacing, an operating frequency must be chosen so that eddy currents will be generated in the second (subsurface) layer. The frequency chosen should produce eddy currents that penetrate both metals to a depth of 3δ. When the gap is zero (both metals touching), the combination appears infinitely thick and produces a vector point on the conductivity locus. As the gap increases, the inductive reactance increases and the phasors produce a locus of points (in the vertical direction) that cross the conductivity locus at some point.

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As the gap increases, the locus of points will also intersect the thinning locus at a point that represents the thickness of the upper metal specimen (Fig. 6).

At this point, the gap has reached a value where eddy currents are being generated only in the upper layer. Hence, any further increase in the gap between the two metal specimens will not cause any further change in impedance. If a larger gap needs to be measured, the operating frequency must be reduced.

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Gap or spacing measurements are best made with the probe located over the thin member; this reduces the problem with depth of penetration.

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If both members are equally thick but one has a lower conductivity, the probe should be placed on the low conductivity specimen.

Note: δ = (ωμσ) - ½ = (πfμσ) - ½

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FIGURE 6. Probe coil impedance curve showing effect of metal spacing between two flat, parallel, metallic conducting sheets.

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18.3

PART 3. Eddy Current Tests of Metal Conductivity

For extending the use of eddy current testing in the control of metal alloys and their heat treatment, the most important prerequisite is the establishment of suitable documentation, test procedures and limits.

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18.3.1

Control of Tests

Important considerations in eddy current conductivity testing are: (1) selection of suitable and reliable instruments, (2) careful laboratory evaluation for defining test requirements and (3) selection of suitable reference standards.

There must be no doubt about the accuracy and reproducibility of instrument performance from one test to the next or between two or more instruments of the same type and from the same manufacturer.

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â– Conductivity Instruments Eddy current conductivity meters usually differ with respect to operating frequency, liftoff compensation, temperature compensation, sensitivity, probe size or means of presenting test results. Most conductivity meters can provide results as percentages of the International Annealed Copper Standard. Other instruments can provide conductivity measurements in siemens per meter or as customized displays of signal amplitude or phase angle. Whatever type of instrument is used, reference standards must be used to calibrate the instrument, permitting qualitative judgments on all future test results.

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â– Laboratory Evaluations Laboratory evaluations are an important step in establishing eddy current test procedures. A suitable laboratory evaluation typically involves analysis of test object composition, heat treating times and temperatures and mechanical processing. The laboratory evaluations serve to establish a conductivity value for an alloy, as illustrated in Fig. 7. â– Conductivity Reference Standards Nondestructive test calibration reference standards should be based on some measurable physical characteristic related to the critical properties being measured. This characteristic must be not only measurable but also reproducible. For conductivity testing, reference standards are usually made from a sample of metal with resistivity that is known and thickness that can be considered infinite for purposes of the test. Physical reference standards for conductivity may be obtained from the National Institute of Standards and Technology or may be fabricated.

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FIGURE 7. Relative conductivity of metals and alloys shown by eddy current meter readings. Unified Numbering System (UNS) is used here for wrought aluminum alloys.

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The reference standards are identified by the material’s conductivity, a reciprocal of its resistivity. When testing aluminum alloys, it is necessary to maintain two or more aluminum alloy reference standards.

Typically, one of these reference standards has conductivity in the range of 14.5 to 18.6 MS·m–1 (25 to 32 percent of the International Annealed Copper Standard) and the other standard is in the range of 22.0 to 29.0 MS·m–1 (38 to 50 percent of the International Annealed Copper Standard).

Such reference standards are certified and their accuracy is documented.

Nonferrous conductivity reference standards can be fabricated for instrument calibration. Conductivity standards in the United States are usually referenced to the National Institute of Standards and Technology NIST.

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18.3.2 Factors Affecting Conductivity ■Alloying Elements If a pure metal were tested in the annealed or unstrained condition, a single conductivity value would be obtained, such as the 100 percent value arbitrarily assigned to pure annealed copper. However, most structural metals are alloys consisting of a base metal with several alloying elements added to obtain specific properties. The addition of alloying elements to a pure metal alters its conductivity. Usually, the alloying element provides centers of interference to electron flow. This reduces the alloyed metal’s conductivity below that of the pure metal.

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Although the conductivity of copper is usually given as 58.0 MS·m–1 (100 percent of the International Annealed Copper Standard), care must be exercised when testing copper, because the conductivity is influenced by small additions of impurities or alloying elements. Conductivity is also affected by the metal’s temperature at the time and place of eddy current testing.

Touching the reference standard with fingers can change its temperature and apparent conductivity. The specific resistivity of standard annealed copper changes by 0.4 percent for each Kelvin or degree Celsius of temperature change in tests near room temperature, 20°C (68°F).

Note: 58.0 MS·m–1 (100 percent of the International Annealed Copper Standard).

The specific resistivity of standard annealed copper changes by 0.4 percent for each Kelvin change of temperature w.r.t 20°C (68°F).

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â– Heat Treatment and Mechanical Stresses The conductivity of a particular alloy is further influenced by heat treatment and by stresses introduced in the metal during fabrication. Heat treatment alters crystal or grain structure and the distribution of the alloying elements, both of which affect electron flow. Stress introduced during fabrication provides centers of interference to electron flow. The combined effects of alloying elements (with their permissible tolerances), of heat treatment (with its permissible time and temperature ranges) and of fabrication stress establish a conductivity range instead of a specific conductivity value for any metal alloy and heat treatment condition. It is the function of laboratory evaluations to define these conductivity ranges and their variations under various test conditions.

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■ Laboratory Evaluation of Aluminum Alloys The following are practical considerations for selecting instruments, reference standards and test samples for a laboratory evaluation to establish the conductivity ranges of an aluminum alloy. First, eddy current instruments, probes and reference standards will be required. Instrument performance should be checked for such effects as drift, including that caused by power supply fluctuations and temperature, and for reproducibility of calibration and instrument readings. Calibration standards of known conductivity should be obtained for the approximate conductivity range expected. Test samples should be flat, unclad and uncoated material of sufficient size to eliminate edge effects and of sufficient thickness to eliminate thickness effects. Surface finish roughness up to 6.3 μm (2.5 × 10–4 in.) or oxide coating thicknesses normally encountered on aluminum sheet, plate or bar will not significantly interfere with the eddy current readings, provided the instrument has liftoff compensation up to 0.08 mm (0.003 in.). Some knowledge of variables that influence eddy current measurements helps ensure proper selection of test samples for establishing conductivity values.

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18.3.3

Factors Affecting Conductivity Tests

Any variation from the condition for which eddy current conductivity meters are calibrated may have a significant effect on the accuracy of their measurements. The basic test condition variables have been studied to determine their effects on measurements of true base material conductivities.

Six basic test condition variables are: (1) curved surfaces, (2) edge effect, (3) material thickness, (4) aluminum cladding thickness, (5) liftoff or nonconductive coating thickness and (6) temperature.

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â– Curved Surfaces Eddy current tests on surfaces having small radii of curvature are to be avoided. A curved surface changes the effective probe coil liftoff and the true material mass subjected to eddy current influence. Figure 8 indicates the effect of curved surfaces on typical eddy current measurements. Eddy current laboratory evaluations for establishing conductivities should be made on test objects with flat surfaces. For convex surfaces, a V block is used to maintain parallelism between the probe face and the curved surface. Curved surface standards are made by machining a thick, flat specimen to various radii of curvature.

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FIGURE 8. Influence of cylindrical curved surfaces on conductivity measurements: (a) probe on convex curvature; (b) probe on concave curvature; (c) signals.

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FIGURE 8. Influence of cylindrical curved surfaces on conductivity measurements: (a) probe on convex curvature; (b) probe on concave curvature; (c) signals.

Decreasing the radius of curvature for both will reduce the measured apparent conductivity. The concave surface has more profound effects.

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â– Edges Edge effects can be demonstrated by moving the eddy current probe toward the edge of a test sample and observing the change in instrument reading that results. Figure 9 is an example of instrument change resulting from edge effect. Test samples must be large enough to prevent this edge interference or probes must be shielded to collimate the field. In this particular case, an edge distance of 5 mm (0.2 in.) or more must be maintained to avoid errors.

If parts having a narrow width must be tested, a centering jig or holder should be used to maintain the probe on center. Edge distance curves are used to apply correction factors to conductivity readings on production parts.

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FIGURE 9. Influence of edge effect on conductivity measured by eddy current probe: (a) probe at edge; (b) signal.

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■ Material Thickness Material thickness will affect test results when the depth of eddy current penetration is greater than the material thickness. The eddy currents will penetrate the test object to an effective depth of penetration 3 δ. Various mathematical equations are documented for determining the standard depth of penetration δ, as described elsewhere in this volume and other publications. The skin effect (described elsewhere in this volume) causes the currents to be concentrated near the surface next to the excitation coil. The effect increases, that is, the depth decreases, with increasing test operating frequency, test object electrical conductivity and test object magnetic permeability. The currents decrease exponentially or almost exponentially with depth, depending on test object shape and thickness. In addition, the phase angle of the current becomes increasingly lagging as the depth increases.

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The phase angle lag θ (radian) uses the phase angle of the current density at the surface as a reference angle: (1)

radian (where is f?)

where x is distance (meter) below the surface, δ is the standard depth of penetration (meter), μ is magnetic permeability (Henry per meter) and σ is electric conductivity (in Eq. 1, as a percentage of the International Annealed Copper Standard).

The calculated values at 18.6 MS·m–1 (32 percent of the International Annealed Copper Standard, corresponding to a resistivity of 54nΩ ·m) and 60 kHz yield a value of δ = 0.48 mm (0.019 in.). These values agree with those obtained using a commercial eddy current slide rule.

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Figure 10 shows the general effect on the coil impedance from changes in conductivity, thickness and liftoff. The included angle is defined as the angle, on the impedance plane, between the locus of liftoff changes and the locus of metal thickness changes. This angle increases with plate thickness and becomes important when impedance analysis is used to minimize the effects of liftoff. To avoid thickness effects, the effective depth of penetration 3δ may be used: (2) where f is frequency (hertz) and, for nonmagnetic materials, μ = 4π × 10–7 H·m–1 = 1.26 μH·m–1.

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FIGURE 10. Conductivity variations with changes of material thickness for 18.6 MS·m–1 (32 percent of International Annealed Copper Standard) aluminum tested at 60 kHz: (a) impedance diagram; (b) conductivity versus thickness. Included angles β and θ correspond to phase angles in instrument display.

Legend β = included angle difficult to interpret δ = standard depth of penetration δA = 1.40 mm (0.055 in.) δB = 0.89 mm (0.035 in.) δC = 0.66 mm (0.026 in.) δD = 0.48 mm (0.019 in.) δE = 0.36 mm (0.014 in.) θ = included angle easy to interpret

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FIGURE 10. Conductivity variations with changes of material thickness for 18.6 MS·m–1 (32 percent of International Annealed Copper Standard) aluminum tested at 60 kHz: (a) impedance diagram; (b) conductivity versus thickness. Included angles β and θ correspond to phase angles in instrument display.

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Whatever the test material is, it must be remembered that the standard depth of eddy current penetration δ is a function of material conductivity, operating frequency and magnetic permeability. Because all these values are in the denominator of the depth of penetration equations, an increase in any of these values will result in a shallower depth of penetration. With fixed frequency instruments, not much can be done, except for stacking, to eliminate the influence of thickness variations on the test results. However, if variable frequency instruments are available, then the operating frequency can be altered to match the effective depth of penetration 3δ value and eliminate errors caused by thickness variations.

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■ Cladding Thickness on Aluminum Alloy Sheet Cladding with commercially pure, high conductivity aluminum on aluminum alloy base materials (which have lower electrical conductivities) causes a greater positive deviation in apparent conductivity as the cladding thickness is increased. Figure 11 shows typical readings of the thickness of commercially pure aluminum cladding on base sheets of Unified Numbering System A97075 and A92024 wrought aluminum alloys. The ordinate (vertical) scale of Fig. 11 shows the change in apparent conductivity of the base metal, resulting from the higher conductivity of the aluminum cladding layer. In typical cases, the cladding adds only about 5 percent to the total sheet thickness but results in an apparent conductivity change of 2.3 to 4.1 MS·m–1 (4 to 7 percent of the International Annealed Copper Standard).

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FIGURE 11. Conductivity change as function of aluminum alloy cladding thickness at 60 kHz: (a) Unified Numbering System A97075 wrought aluminum alloy base material; (b) Unified Numbering System A92024 wrought aluminum alloy base material.

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FIGURE 11. Conductivity change as function of aluminum alloy cladding thickness at 60 kHz: (a) Unified Numbering System A97075 wrought aluminum alloy base material; (b) Unified Numbering System A92024 wrought aluminum alloy base material.

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Eddy current conductivity testing (at 60 kHz) for acceptance of aluminum clad material is usually restricted to material less than 2.3 mm (0.09 in.) in total thickness. Testing of thicker materials requires removal of most of the aluminum cladding layer by spot facing a small area and taking a reading there. This reading is taken on a small percentage of parts in the lot. A correction factor is determined between the spot faced areas and those where the cladding has not been removed. The remaining parts are checked without spot facing and the correction factor is applied to the readings.

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■ Compensation for Probe Coil Liftoff on Coated Metals The liftoff is the distance that the conductivity meter probe coil is held away from the metal surface by a nonconductive film. The liftoff setting of the test instrument is made by the liftoff control available on some eddy current instruments. This adjustment is made so that true conductivity readings for the base metal are the same as the readings made on a base metal covered by a thin, nonconductive film.

Thus, when the liftoff control is set for a specific film thickness on metal with a conductivity of 17.7 MS·m–1 (30.5 percent of the International Annealed Copper Standard), the conductivity of a metal covered with a paint layer of that thickness will be read as 17.7 MS·m–1 (30.5 percent of the International Annealed Copper Standard).

During tests of alloys with different conductivities and with varying thicknesses of nonconductive coatings, it would be necessary to correct the apparent conductivity readings for the deviations caused by liftoff effects to obtain true readings for the base metal conductivities.

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â– Test Material Temperatures Temperature is an important test variable, particularly when eddy currents are used to establish a basic conductivity range for an alloy. Consideration must be given to: (1) the temperature of the test material, (2) the difference in temperature between the test sample and reference standards used for calibration and (3) the type of eddy current instrument being used.

The influence of temperature on the resistivity of a metal can be determined from Eq. 3: (3) where Rt is resistivity (ohm meter) of the metal at the test temperature, R0 is resistivity (ohm meter) of the metal at standard temperature, ΔT is the difference between the standard and test temperatures (Kelvin) and ι is the resistivity temperature coefficient (rarely negative).

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From Eq. 3 it can be seen that if the temperature is increased, resistivity increases and conductivity decreases from their ambient temperature levels.

Conversely, if temperature is decreased, the resistivity decreases and conductivity increases.

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18.4 PART 4. Eddy Current Testing of Bolt Holes Small cracks in material next to fastener holes may go undetected until the cracks grow to a size that allows detection at a surface not covered by the head of the fastener or nut. Unfortunately, in many highly stressed areas, a crack of this magnitude may continue to grow to failure at a rapid rate. Eddy current testing is a reliable way to detect cracks in material adjacent to fastener holes. Eddy current tests are well known for detecting small fatigue cracks, particularly in aluminum fastener holes. Automated bolt hole scanners provide reliable and repeatable test results. Finding small cracks at fastener holes sometimes requires removing the fastener and performing an eddy current check. Automatic scanning may be used to detect cracks as small as 0.13 mm (0.005 in.).

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Bolt Holes

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http://archives.sensorsmag.com/articles/0303/28/main.shtml

Bolt Holes

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http://archives.sensorsmag.com/articles/0303/28/main.shtml

Figure 12 illustrates an eddy current bolt hole probe. The probe coil axis is perpendicular to the material adjacent to the hole. An adjustable collar is used to position and locate the coil a desired distance inside the hole. Usually, testing is started with the coil just below the test material surface and the probe is manually rotated 360 degrees. After each rotation, the probe is advanced in increments to ensure adequate overlap and coverage. The spherical end of the probe is often split in the middle so that a small rubber or plastic V wedge may be inserted to make the probe snug in oversized holes.

These wedges are not generally used for probes less than 6.3 mm (0.25 in.) in diameter.

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FIGURE 12. Eddy current hole probe.

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During bolt hole testing, the absolute technique is generally used. The absolute technique uses one coil on a ferrite core; measurements are made while the material is in direct contact with the probe coil. The differential technique uses two coils and compares measurements obtained from the (uncracked) material under one coil with the (cracked) material under the other coil. A bridge circuit is unbalanced if one coil is over sound material and the second coil is over a discontinuity. Figure 13 shows crack responses typical of bolt hole probes inserted in an aluminum sample: a single-loop response from an absolute bolt hole probe (Fig. 13a) and a double-loop response from a differential probe (Fig. 13b).

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FIGURE 13. Normalized impedance graph showing crack responses for absolute and differential bolt hole probes: (a) absolute probe crack response at 100 kHz; (b) differential probe crack response at 500 kHz.

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FIGURE 13. Normalized impedance graph showing crack responses for absolute and differential bolt hole probes: (a) absolute probe crack response at 100 kHz; (b) differential probe crack response at 500 kHz.

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18.4.1 Reference Standards for Bolt Hole Testing Reference standards aid in calibrating or adjusting instrument controls and are used to ensure that cracks will be detected with a predetermined sensitivity, based on depth, length and location (see Fig. 14).

Reference standards have been fabricated by simulating cracks with electric discharge machined radial slots of various depths and lengths (Fig. 15). This type of reference standard works well if the notches are less than 0.13 mm (0.005 in.) wide. However, simulated discontinuities do not respond exactly the same as real fatigue cracks. Some investigators have produced cracks by introducing an electric discharge machined slot in the hole and then subjecting the specimen to fatigue loading.

A crack initiates and grows outward from the machined slot. The depth and length of the crack are qualitatively controlled by the stress and the number of cycles. After the crack has grown to the approximate size, the electric discharge machined slot is removed by drilling the hole to a sufficient, predetermined oversize. The depth of the resulting crack is estimated; the true depth can be determined only by fracturing the reference standard.

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FIGURE 14. Typical location of cracks adjacent to fastener holes: (a) at top; (b) at middle of hole; (c) at bottom; (d) on beveled surface.

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FIGURE 15. Calibration block for bolt hole and surface probe tests.

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Calibration block for bolt hole and surface probe tests.

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Calibration block for bolt hole and surface probe tests.

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Calibration block for bolt hole and surface probe tests.

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18.4.2 Procedure for Bolt Hole Testing A typical procedure for performing eddy current bolt hole tests is listed below.

1. Determine the diameter and depth of holes to be tested from engineering drawings or other applicable documents.

2. Select sizes of probes and of reference standards.

3. Calibrate the instrument response to the reference standards.

4. Visually check holes for excess corrosion or galling. Remove material from a hole by manually passing a reamer through the hole or drilling out to next oversize diameter.

5. Scan for discontinuities by rotating the probe at constant depths, at intervals not exceeding 1.5 mm (0.06 in.) deep. Conduct scanning in intervals of about one revolution each. If suspect areas are noted near the ends of scans, retest these areas so that the suspect areas are in the middle of the second scans.

Fatigue crack depths may be estimated from the amplitudes of eddy current readings. When observing meter deflections, make sure the deflection is not caused by scratches, surface irregularities, out-of-roundness of the hole or tilting of the probe.

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â– Bolt Hole Probes Bolt hole probes are generally available from eddy current equipment manufacturers. It must be remembered that different types of probes are usedfor testing straight or tapered holes and that different collars are used for flat, countersunk or curved outer surfaces. â– Strip Chart Recorders Although many bolt hole tests are performed manually (where the operator directly observes the meter deflection or screen presentation), strip charts or computers may be used to process, record or interpret the test results.

Signals produced by various discontinuities are shown in Fig. 16. The trace has been rotated to make the air-to-aluminum line horizontal as in Fig. 13a.

The vertical output is then recorded by a slow running strip chart recorder.

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FIGURE 16. Strip chart indications obtained from typical discontinuities during eddy current bolt hole test: (a) scratch; (b) crack or corrosion; (c) corrosion; (d) conductivity variation; (e) non-perpendicular hole axis. Deflections to left indicate downscale meter deflections.

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18.4.3 Automated Bolt Hole Testing When numerous bolt holes need to be tested, it is often desirable to eliminate manual scanning and use a device that automatically rotates the probe in the hole. Eddy current equipment manufacturers have developed such systems.

A system has been designed for the testing of shallow holes and tube ends.12 The probe is attached to the end of a short spindle and is rotated in a hand held, rotating head. The United States Air Force has also developed bolt hole tests. An automatic eddy current discontinuity detection system detects and records discontinuities on conductive metal surfaces and in structural fastener holes, reducing the errors of conventional hand scanning.

The system can detect cracks as small as 0.25 mm (0.010 in.) long by 0.13 mm (0.005 in.) deep in aluminum. Scan depths are adjustable from 6.3 to 3.8 mm (0.25 to 1.50 in.). The scanner’s speed can be varied from 0 to 2.45 Hz (0 to 150 rotations per minute), providing simultaneous linear and rotational (spiral) motion. Either filtered or unfiltered signals from the scanner can be displayed on a chart recording or oscilloscope. The analog recorder provides a detailed profile of discontinuities for analysis. The system is fully portable and all equipment is contained in a protective case.

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Other rotating eddy current test systems are available for rapidly detecting small cracks in aircraft bolt holes. They use small, light, hand held scanners and various sizes of bolt hole probes. These instruments are simplified, easy to use and portable. They generally operate at 500 kHz and use differential probes rotating at 50 cycles per second (3000 rotations per minute). The resolution of the differential scanning coil system follows the physical laws for general eddy current testing; when the probe moves on an orthogonal path over a crack, the crack depth is the principal factor affecting the amplitude, provided its length is equal to or greater than the coil diameter.

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The main application is the rapid testing of fastener holes for cracks. The high revolution speed of 50 cycles per second (3000 rotations per minute) of the rotating probe and the clear display on the screen permit a high test sensitivity. The saving of time compared to the hand held technique is about 80 percent. In principle, all metals can be tested, including titanium, aluminum and steel.

The detection sensitivity depends on the roughness of the surface and begins at about 0.20 mm (0.008 in.) depth.

Two different kinds of display may be used: Y,t with one sweep per revolution (Fig. 17a) and the X,Y vector for differentiation of the signal in phase and amplitude (Fig. 17b). The lightness and small size of the detection probe allows the instrument to be used in confined areas.

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FIGURE 17. Cathode ray tube displays: (a) amplitude versus time; (b) lissajous phase analysis presentation of X,Y components on impedance plane.

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FIGURE 17. Cathode ray tube displays: (a) amplitude versus time; (b) lissajous phase analysis presentation of X,Y components on impedance plane.

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Eddy Current Bolt Hole Testing

https://www.youtube.com/embed/dUIa1aVE9fc

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Eddy Current Bolt Hole Testing

https://www.youtube.com/embed/X4yqOLUYrBs Charlie Chong/ Fion Zhang

Eddy Current Bolt Hole Testing

https://www.youtube.com/embed/o7aBvCsWX8Q Charlie Chong/ Fion Zhang

18.5 PART 5. Impedance Plane Analysis of Typical Aerospace Material Tests 18.5.1 Impedance Plane Eddy current probes typically consist of one or more wire wound coils that behave electromagnetically as inductors. Thus, the probe coils possess both an inductive reactance and a resistance. These two values combine to produce the overall electrical impedance of the probe. Instruments that measure impedance provide more complete test data during eddy current testing. Because electrical impedance contains two components, it can be represented as a two-dimensional graph such as that shown in Fig. 18. This graph is called the impedance plane. The coil resistance R is plotted horizontally and its inductive reactance X is plotted vertically.

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FIGURE 18. Impedance changes in relation to one another for surface probes of 6.3 mm (0.25 in.) diameter.

Legend C = crack in aluminum Pa = plating aluminum on copper Pc = plating copper on aluminum Pn = plating nonmagnetic S = spacing between aluminum layers T = thinning in aluminum ﾎｼ = permeability ﾏハ = conductivity nonmagnetic materials

During eddy current testing, a relative impedance point (material test point) is obtained by placing the probe against the sample and setting the bridge controls for X and R. These X and R values (phasors) are then traced on the instrument screen display. Each eddy current measurement of a test variable generally possesses a unique location on the normalized impedance graph.

Different material conditions —such as conductivity, permeability, discontinuity, liftoff, spacing and thinning — produce their own unique impedance plane plots. The diversity of plots allows rapid, significant impedance plane analysis in a wide variety of applications.

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Continuously changing the magnitude of a given variable traces out an impedance curve (locus of points) that is a characteristic signature for that variable. The amount of movement and the position of a test point along the curve is related to the magnitude of the impedance change. This analysis of impedance components is the basis for eddy current signal interpretation.

Eddy current phase analysis instruments enable the operator to produce impedance plane traces automatically on the integral X,Y storage oscilloscope.

These instruments operate from 60 Hz to 6 MHz, allowing the operator to choose the best frequency for a given material and test. For the eddy current test response to different conductivities of various nonmagnetic alloys, the material points trace out a characteristic comma shaped curve with conductivity increasing in a clockwise direction. The figure also shows probe liftoff impedance curves for two of the alloys (liftoff is the spacing between the probe and the bare metal surface).

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The material test points move clockwise around all impedance curves in a nonlinear fashion as test material conductivity and test frequency are increased. Figure 18 shows impedance plane responses for several other types of common variables.

Examples of metal thinning are shown in Figs. 1 and Figs. 10. An impedance chart of cladding thickness is shown in Fig. 3 and an impedance chart of liftoff, for both magnetic and nonmagnetic materials, is shown in Fig. 4. The impedance plane response for separation (spacing) between two aluminum sheets is shown in Fig. 6. The impedance response for surface cracks in aluminum is shown in Fig. 13.

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Examples of:  metal thinning are shown in Figs. 1 and Figs. 10.  An impedance chart of cladding thickness is shown in Fig. 3 and  an impedance chart of liftoff, for both magnetic and nonmagnetic materials, is shown in Fig. 4.  The impedance plane response for separation (spacing) between two aluminum sheets is shown in Fig. 6.  The impedance response for surface cracks in aluminum is shown in Fig.13.

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18.5.2

Test Frequency Selection

One of the most important considerations in eddy current testing is the choice of test frequency. The type of alloy involved and the variables to be measured or suppressed determine the best frequency. The depth of eddy current penetration within test materials is strongly affected by test frequency. For a given alloy, higher frequencies normally limit the eddy current test to inspection of the excited metal surface nearest the magnetizing coil winding.

Lower frequencies permit deeper eddy current penetration. A given test frequency will allow eddy currents to penetrate deeper in lower conductivity alloys than in higher conductivity alloys.

High test frequencies are normally used for detecting small surface cracks or surface contamination and for gaging thin coatings. Medium frequencies are useful for conductivity measurements such as alloy sorting. Low test frequencies are usually required for testing thicker materials (for opposite side corrosion, for example), for thickness gauging and for penetrating into magnetic materials.

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Penetration depth, however, is only part of the process for selection of optimum eddy current test frequencies. The geometric relationship between the impedance curves for the variable to be measured and the variable to be suppressed must also be considered. Furthermore, the position of the variable magnitudes (different conductivity points or liftoff points) along their respective impedance curves is important.

Test frequency affects the conductivity and liftoff curves of nonmagnetic alloys. Changes in frequency shift the points along the curves in a nonlinear fashion. This phenomenon, also true for other impedance curves (including magnetic materials), can be advantageously used because it allows the material points to be positioned for optimum response to desired variables and for suppression of undesired variables.

Specifically, a frequency should be chosen that causes the impedance curve points (material points) for the variable to be measured to move in a substantially different direction than those points for the variable to be suppressed.

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At low frequencies, the angle between the liftoff curve and the region of the conductivity curve for the titanium point is quite small. Thus, it becomes more difficult to obtain liftoff suppression without a noticeable expense to the response for conductivity variations or surface cracks in titanium. By selecting a higher frequency, the angle between titanium test points and liftoff is quite large, which allows liftoff suppression and good sensitivity to conductivity variations in the titanium.

Conductivity is measured to identify alloys and heat treatment conditions. For conductivity measurements and for surface crack detection, a frequency in the conductivity curve should be chosen that places the material test point just below the point of maximum coil resistance and eddy current losses (the region of highest sensitivity to changes in all test variables).

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This location for the material test point creates a desirably large angle between the liftoff and conductivity responses, allows good liftoff suppression and provides good sensitivity to conductivity variations.

The material points for all other variables (metal thickness, plating thickness and others) are also shifted clockwise around their impedance curves by frequency increases. Therefore, a rationale similar to that discussed above applies to all variables.

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18.5.3

Applications of Impedance Plane Analysis

Examples of metal thinning caused by corrosion include the following: (1) metal spacing between an aluminum and titanium skin in an engine nose cowl, (2) localized metal thinning of a chemically milled step and (3) surface crack detection in aircraft structure. â– Metal Thinning from Corrosion Moisture entrapment between an aluminum fuselage belly skin and a titanium doubler causes corrosion thinning in the skin. To detect the corrosion and determine metal thinning, the inspector can be given a sketch showing the location and outline of the internal doubler. The skin material and temper can be identified along with the electrical conductivity values. An aluminum thickness taper gage (Fig. 19a) of alloy similar to the skin material is used to calibrate the eddy current instrument for metal thinning.

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The probe is placed on the taper gage at the 1.3 mm (0.05 in.) thickness and the response shown in Fig. 19b is obtained. The probe is then slowly scanned to the thin end of the gage and is lifted off at 0.2 mm (0.01 in.) thickness, yielding the response shown in Fig. 19c. Thickness calibrations are performed by rescanning the taper gage and lifting off the probe at various thickness increments as shown in Fig. 19d. The instrument is now calibrated for the test.

The operator looks for corrosion thinning while the aluminum external skin is scanned over the area of the internal doubler. If there is no corrosion, the response shown in Fig. 20b will be obtained. Severe corrosion will yield a response similar to that shown in Fig. 20c. The depth of the corrosion is determined by using the taper gage to measure the depth from maximum amplitude response as shown in Fig. 20d. In this case, the corrosion thinning is 0.63 mm (0.025 in.), which represents 50 percent of the skin thickness.

Cracks may be associated with severe corrosion and, if they exist, the response shown in Fig. 20e will be obtained.

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FIGURE 19. Screen display calibration at 20 kHz for 1.3 mm (0.05 in.) thick skins: (a) aluminum taper gage; (b) taper gage signal at 1.3 mm (0.05 in.) thickness; (c) taper gage signal at 0.2 mm (0.01 in.) thickness; (d) completed thickness calibration using taper gage.

Voltage measurement increment is assigned during calibration.

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FIGURE 19. Screen display calibration at 20 kHz for 1.3 mm (0.05 in.) thick skins: (a) aluminum taper gage; (b) taper gage signal at 1.3 mm (0.05 in.) thickness; (c) taper gage signal at 0.2 mm (0.01 in.) thickness; (d) completed thickness calibration using taper gage.

Voltage measurement increment is assigned during calibration.

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FIGURE 20. Impedance plane screen display presentations for exfoliation corrosion metal thinning: (a) diagram; (b) screen display presentation for no corrosion; (c) screen display presentation for severe corrosion; (d) method for determining depth of corrosion; (e) surface crack response in corroded area.

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FIGURE 20. Impedance plane screen display presentations for exfoliation corrosion metal thinning: (a) diagram; (b) screen display presentation for no corrosion; (c) screen display presentation for severe corrosion; (d) method for determining depth of corrosion; (e) surface crack response in corroded area.

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FIGURE 20. Impedance plane screen display presentations for exfoliation corrosion metal thinning: (a) diagram; (b) screen display presentation for no corrosion; (c) screen display presentation for severe corrosion; (d) method for determining depth of corrosion; (e) surface crack response in corroded area.

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â– Metal Spacing During fabrication of engine nose cowl inlet ducts, problems have been encountered in maintaining a gap between the outer aluminum skin and inner titanium skin for the purpose of hot air deicing. An eddy current technique was developed and used to evaluate the gap at various locations around the nose cowl. Results indicated that a consistent spacing was not being maintained and that the gap spacing was below minimum tolerance in some areas of the duct. Because of this, aluminum button spacers were installed between the aluminum and titanium skins to make the gap consistent. Before taking gap thickness measurements, a reference standard was fabricated from a plate of 1.5 mm (0.06 in.) thick Unified Numbering System A92219 wrought aluminum alloy (Fig. 21) to represent the outer skin of the cowl.

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The 0.5 mm (0.02 in.) thick commercially pure titanium shims represent the internal skin of the cowl. The nonconductive acrylic step wedge represents the air gap or space between the inner and outer skins. The instrument as calibrated by placing the probe on the low conductivity (titanium) side of the standard at the zero and 2.5 mm (0.10 in.) gap positions. The frequency and instrument settings varied until a maximum spread was obtained on the screen display.

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FIGURE 21. Eddy current reference standard for spacing measurement: (a) diagram of use; (b) top view; (c) side view.

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FIGURE 21. Eddy current reference standard for spacing measurement: (a) diagram of use; (b) top view; (c) side view.

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FIGURE 21. Eddy current reference standard for spacing measurement: (a) diagram of use; (b) top view; (c) side view.

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â– Localized Metal Thinning In one case, a thinning of a chemically milled step in a bonded honeycomb wing tip was discovered. A test was needed to both detect and measure the thickness at the discrepant area of the Unified Numbering System A97075, temper 6, wrought aluminum alloy skin. The skin thickness ranged from 0.45 to 0.70 mm (0.018 to 0.028 in.) in the chemically milled area. Reference standards 0.41, 0.46 and 0.51 mm (0.016, 0.018 and 0.020 in.) thick were used to calibrate the instrument at 20 kHz. The liftoff was rotated on the screen display by a phase adjustment so that it was displayed in the horizontal direction. The wing tips were measured by placing the probe at the inboard or thin part of the bonded panel and scanning outboard across the chemically milled step (Fig. 22).

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FIGURE 22. Eddy current impedance plane testing of bonded honeycomb panel in wing tip.

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When the test object had a normal chemically milled step, the screen display trace shown in Fig. 23a was obtained. When the part had local undercut in the chemically milled step, then the trace shown in Fig. 23b was obtained. By marking the screen display with the minimum 0.46 mm (0.018 in.) thickness during calibration, the depth of the undercut could be determined.

Numerous test objects were evaluated and test objects showing less than minimum thickness were rejected and sectioned and the eddy current results were verified.

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FIGURE 23. Eddy current impedance plane results from inspection of wing tip: (a) results from normal chemically milled step; (b) results from undercut chemically milled step.

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â– Exfoliation Corrosion around Wing Skin Fastener Holes Although contact between galvanically dissimilar metals, such as steel and aluminum, is known to be a cause of corrosion, the design of aircraft structures and systems occasionally requires that such metals be joined together. When dissimilar metals must be joined, it is a design requirement that the contacting surfaces be electrically insulated with organic paint or sealant or that one of the surfaces be coated with a metallic coating galvanically similar to the other surface.

For example, cadmium plated steel bolts are used on many aircraft. The cadmium plating protects the steel bolt from corrosion, provides a surface galvanically similar to aluminum and reduces the problem of accelerated corrosion. If the cadmium is depleted or a crevice where moisture can collect exists between the fastener head and the aluminum skin, pitting and intergranular corrosion may occur.

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Intergranular corrosion occurs along aluminum grain boundaries, which in sheet and plate are oriented parallel to the surface of the material as a result of the rolling process. Intergranular corrosion, called exfoliation corrosion in its more severe form, is characterized by delamination of thin layers of aluminum parallel to the sheet surface, with white corrosion products between the layers. Where fasteners are used, the corrosion extends outward from the fastener hole, either from the entire circumference of the hole or in one direction from a segment of the hole. In severe cases, the surface bulges upward. In less severe cases, there may be no telltale bulging: corrosion may be detected only by nondestructive testing.

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Eddy current tests for exfoliation corrosion are shown in Fig. 24. A phase sensitive instrument providing an impedance plane display was operated at 20 and 50 kHz. The depth of penetration of the eddy currents is a function of operating frequency and material conductivity. The effective depth of penetration in the skins, clad with Unified Numbering System A97075 wrought aluminum alloy, temper 6, was about 1.3 mm (0.05 in.) at 50 kHz and 2.3 mm (0.09 in.) at 20 kHz. The conductivity of the coated aluminum skins ranged from 26.1 to 29.0 MS¡m–1 (45 to 50 percent of the International Annealed Copper Standard).

The phase sensitive eddy current instrument was calibrated to yield a maximum response difference between a corroded and uncorroded area.

Figure 24 shows the screen display response from an uncorroded area (Fig. 24b), from a corroded area with 20 kHz and a 13 mm (0.5 in.) diameter probe (Fig. 24c) and from a corroded area with a 50 kHz, 6.3 mm (0.25 in.) diameter probe (Fig. 24d). Identical responses were obtained with and without a steel fastener in the hole.

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The phase angle between the liftoff and corrosion response was about 90 degrees at 50 kHz and 45 degrees at 20 kHz (Fig. 24). Because of the greater phase angle response at 50 kHz, remaining tests were conducted at this frequency. The test was performed by scanning 360 degrees around the periphery of each hole. Corroded holes were marked and the extent of the corrosion was outlined on the skin surface with a marking pen.

If corroded samples are not available to calibrate the instrument before testing, a taper gage reference standard (Fig. 19a) may be made from similar material. The instrument is null balanced with the probe on the thick portion of the standard. After calibration, a corroded area will yield a response similar to that shown in Fig. 24d. A circle template is generally used to guide the probe as it is scanned 360 degrees around each fastener.

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FIGURE 24. Eddy current cathode ray tube responses for exfoliation corrosion around fastener holes in wing skins: (a) diagram; (b) eddy current response, no corrosion; (c) eddy current response from corrosion using 20 kHz and 13 mm (0.5 in.) diameter probe; (d) eddy current response from corrosion using 50 kHz and 6.3 mm 0.25 in.) diameter probe.

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FIGURE 24. Eddy current cathode ray tube responses for exfoliation corrosion around fastener holes in wing skins: (a) diagram; (b) eddy current response, no corrosion; (c) eddy current response from corrosion using 20 kHz and 13 mm (0.5 in.) diameter probe; (d) eddy current response from corrosion using 50 kHz and 6.3 mm 0.25 in.) diameter probe.

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FIGURE 24. Eddy current cathode ray tube responses for exfoliation corrosion around fastener holes in wing skins: (a) diagram; (b) eddy current response, no corrosion; (c) eddy current response from corrosion using 20 kHz and 13 mm (0.5 in.) diameter probe; (d) eddy current response from corrosion using 50 kHz and 6.3 mm 0.25 in.) diameter probe.

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FIGURE 24. Eddy current cathode ray tube responses for exfoliation corrosion around fastener holes in wing skins: (a) diagram; (b) eddy current response, no corrosion; (c) eddy current response from corrosion using 20 kHz and 13 mm (0.5 in.) diameter probe; (d) eddy current response from corrosion using 50 kHz and 6.3 mm 0.25 in.) diameter probe.

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18.5.4

Surface Crack Detection

Eddy current testing is used to find cracks in aircraft structures during inservice maintenance checks. Impedance plane analysis provides a better understanding of crack response loci with other variables on the impedance plane. The value of this approach is evident in Fig. 25a where the crack response locus is 90 degrees out of phase with the liftoff locus for Unified Numbering System G43400 nickel chrome molybdenum alloy steel. Because of the wide separation angle between these two loci, cracks in steel are fairly easy to detect. The lower amplitudes and phase angles for electric discharge machined notches in the steel sample are also shown.

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Figure 25b shows that for aluminum alloys, the electric discharge machined notch and crack response lie between the liftoff and conductivity loci. The smaller notch response almost parallels the liftoff response but the angle of separation and amplitude increases as the notch depth increases. A fairly good response is obtained from a deep crack in a similar aluminum specimen.

It can be seen in Fig. 25b that the crack response is almost parallel to the conductivity locus. This characteristic is true for all nonmagnetic and magnetic materials, as illustrated in Fig. 26.

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Skin cracks, which initiated at fastener holes in a fuselage splice, grew by atigue in a circumferential direction. Flush head aluminum rivets joined the fuselage skin to the frames and longerons. Detection of the surface cracks was possible with standard eddy current crack depth meters. These instruments required scanning between each row of fasteners, which was somewhat time consuming. To speed up the test, a procedure (operating at 5 kHz) was developed for phase analysis screen display instrumentation. Using this type of equipment, the operator can scan in a direct circumferential line over the installed fasteners as shown in Fig. 27a. In Fig. 26, crack responses are in the same direction as the conductivity locus and therefore should be in the same direction as the change in conductivity caused by the rivets. Figure 27b shows the response obtained by scanning over the rivets with no cracks in the skin. In Fig. 27c, the crack response is in the same direction but has a much greater impedance, resulting in a clearly discernible increased amplitude signal.

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Airframe Structures- longerons

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Airframe Structures- longerons

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FIGURE 25. Notches and cracks at 200 kHz on impedance plane screen display: (a) electric discharge machined notches and crack in steel; (b) electric discharge machined notches and crack in aluminum.

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FIGURE 25. Notches and cracks at 200 kHz on impedance plane screen display: (a) electric discharge machined notches and crack in steel; (b) electric discharge machined notches and crack in aluminum.

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FIGURE 26. Direction of surface cracks on impedance plane cathode ray tube.

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FIGURE 27. Cracks in first layer initiating at aluminum fastener holes: (a) looking inboard, left side shown, right side opposite; (b) scan over rivets with no crack in skin at 5 kHz using 9.6 mm (0.38 in.) diameter probe; (c) scan over rivets with crack in skin.

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FIGURE 27. Cracks in first layer initiating at aluminum fastener holes: (a) looking inboard, left side shown, right side opposite; (b) scan over rivets with no crack in skin at 5 kHz using 9.6 mm (0.38 in.) diameter probe; (c) scan over rivets with crack in skin.

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18.5.5

Gain Adjustment to Enhance Indications

With phase analysis instruments, the technique called null balancing described earlier makes the test results easier to interpret. Null balancing entails rotating the impedance place to help interpret signals of interest. This technique is sometimes difficult with screen display instruments because the flying dot impedance point will in many cases be off screen.

The onscreen results of null balancing can sometimes be improved by adjusting the gain. As the optimum operating frequency is selected by rotating the signal traces on the impedance plane, the vertical or horizontal gains on the screen display can be increased or decreased. The traces are rotated by turning the phase control on the face of the instrument. At 5 V per division, the gain is low on the screen display; at 0.2 V per division, the gain is high on the screen display. Two examples are shown in Fig. 28. The first example shows the normal impedance plane response for a crack in an aluminum sample (Fig. 28a).

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The liftoff locus is rotated about 45 degrees to a horizontal position as shown in Fig. 28b. Because the liftoff is an unwanted response, the gain in the horizontal direction is reduced by using a setting of 2 V per division on the screen display. To amplify the crack response in the vertical direction, the vertical gain is increased. This change in gain amplifies both the separation angle (between liftoff and crack response) and the amplitude of the crack response. Figure 28b shows the response that can be expected with differential gain settings superimposed for the same frequency response.

The second example is for a crack in steel as normally presented on the impedance plane (Fig. 28c). Here again, the liftoff response is rotated to a horizontal direction with the crack response in the vertical direction. The horizontal gain is set at 2 V per division and the vertical gain is increased to 0.2 V per division (see Fig. 28d). In this case, the phase angle does not change appreciably but the signal amplitude is increased considerably at the 0.2 V per division. These two examples are typical of situations where null balancing reduces the irrelevant signal amplitude while increasing the signal amplitude of interest.

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FIGURE 28. Improving test results at 50 kHz by changing screen display vertical gain: (a) normal impedance plane response from crack in aluminum; (b) crack responses at various changes in vertical gain; (c) normal impedance plane response from crack in steel; (d) crack responses at various changes in vertical gain.

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FIGURE 28. Improving test results at 50 kHz by changing screen display vertical gain: (a) normal impedance plane response from crack in aluminum; (b) crack responses at various changes in vertical gain; (c) normal impedance plane response from crack in steel; (d) crack responses at various changes in vertical gain.

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FIGURE 28. Improving test results at 50 kHz by changing screen display vertical gain: (a) normal impedance plane response from crack in aluminum; (b) crack responses at various changes in vertical gain; (c) normal impedance plane response from crack in steel; (d) crack responses at various changes in vertical gain.

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FIGURE 28. Improving test results at 50 kHz by changing screen display vertical gain: (a) normal impedance plane response from crack in aluminum; (b) crack responses at various changes in vertical gain; (c) normal impedance plane response from crack in steel; (d) crack responses at various changes in vertical gain.

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18.5.6

Subsurface Crack Detection

Engineers for two aircraft manufacturers investigated subsurface crack detection in aircraft structures using low frequency eddy current techniques.

Figure 29 shows how cracks through the thickness of second and third layer aluminum structures appear on the impedance plane at 1 kHz. The surface crack response in the first layer runs almost parallel to the liftoff locus. The second layer crack runs parallel to the conductivity locus and the third layer crack response is to the right of the conductivity locus. The phase lag causes the phase shift of the crack signals as the crack depth below the surface increases.

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FIGURE 29. Direction of surface and subsurface crack indications in aluminum on impedance plane: (a) test setup at 1 kHz; (b) impedance plane with enlargement of circled area.

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FIGURE 29. Direction of surface and subsurface crack indications in aluminum on impedance plane: (a) test setup at 1 kHz; (b) impedance plane with enlargement of circled area.

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18.6

PART 6. Low Frequency Eddy Current Testing of Aircraft Structure

Airlines and airframe manufacturers have been using eddy current crack detection techniques since the early 1960s. Most of the eddy current equipment and procedures were designed to operate at high frequencies (10 kHz to 1 MHz) to detect very small surface cracks. Around 1973, it was determined by damage tolerance studies that eddy current testing was more sensitive than radiography for detecting fatigue cracks.

It then became desirable to use eddy current testing instead of radiography to detect subsurface cracks. Unfortunately, at that time, instruments operating at low frequencies (100 Hz to 10 kHz) were scarce and, because the probes were large in diameter, they lacked the sensitivity to detect small cracks. Low frequency eddy current testing became reliable with the introduction of phase analysis instruments. In addition, instrument manufacturers lowered the operating frequency to 60 Hz and reduced the size of the probes.

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18.6.1

Depth of Penetration

The depth of penetration of eddy currents below the test surface is influenced by operating frequency, material conductivity and magnetic permeability.

These three variables are used to define the standard depth of penetration δ, the depth at which the eddy current strength or intensity is reduced to 37 percent of its surface value.

The relative magnetic permeability is 1.0 for nonmagnetic materials and the conductivity of a particular material is generally known and constant. Hence, the depth of penetration is controlled by the operating frequency. The δ value is used when the eddy currents need to penetrate a faying surface layer to detect anomalies in a second layer. If the inspector does not want the thickness variations of the test object to influence the test, the effective depth of penetration 3 δ is used.

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The effective depth of penetration is the point at which the eddy current density is reduced to about 5 percent of its value at the surface (and thickness effects are no longer noticed). Figure 30 is a plot of standard depth of penetration δ versus operating frequency for (1) titanium, (2) Unified Numbering System S30400 austenitic chromium nickel stainless steel, (3) Unified Numbering System A97075 wrought aluminum alloy and (4) high alloy steel.

By using these data, δ values or operating frequencies may be obtained for each application. (A chart as in Fig. 5 may also be used.)

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FIGURE 30. Eddy current standard depth of penetration for various conductivity materials at different frequencies.

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â– Applications for Low Frequency Testing Low frequency eddy current techniques have been used successfully for detecting hidden corrosion (see Fig. 31). The areas that are most prone to corrosion are those that collect moisture either by leakage or by condensation.

Once the corrosion starts, it becomes intergranular and causes continuous exfoliation until the corrosion propagates all the way through the member. If the corrosion products collect between faying surfaces, a blister may form and cause interface separation or spacing. Thinning or spacing effects can be detected and measured by low frequency eddy current testing. Low frequency eddy current techniques have detected corrosion in jet aircraft.

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FIGURE 31. Typical applications for low frequency eddy current testing: (a) interstitial corrosion; (b) thinning; (c) second layer cracking; (d) cracking in fastener hole.

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FIGURE 31. Typical applications for low frequency eddy current testing: (a) interstitial corrosion; (b) thinning; (c) second layer cracking; (d) cracking in fastener hole.

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Intergranular Corrosion

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http://www.nace.org/Corrosion-Central/Corrosion-101/Intergranular-Corrosion/

Intergranular Corrosion

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http://www.nace.org/Corrosion-Central/Corrosion-101/Intergranular-Corrosion/

An operating frequency of 4.5 kHz was used, producing a standard depth of penetration of 1.8 mm (0.07 in.), sufficient to guarantee penetration into the second member. A shielded 9 mm (0.35 in.) diameter probe was designed to minimize edge effects. Reference standards to simulate spacing or thinning were produced by end milling flat bottom holes in an aluminum skin. A spacing of 0.5 mm (0.02 in.) produced a full scale vertical movement of the flying dot and the 0.13 mm (0.005 in.) gap caused a proportionally smaller movement. Gaps greater than 0.5 mm (0.02 in.) were considered serious corrosion and gaps between 0.25 mm (0.010 in.) and 0.51 mm (0.020 in.) were considered suspect.

This example is similar to one discussed above (Fig. 20). Low frequency eddy current equipment operating at 500 Hz has been used19 to detect 1.0 mm (0.04 in.) subsurface cracks in aluminum structure through aluminum layers 5 mm (0.2 in.) thick. This test technique has been effective for detecting cracks in second layer structure and has detected cracks missed by radiography.

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Although the low frequency, 500 Hz eddy current technique is effective, it has several limitations.

1. It cannot be used to test through outer layers thicker than 6.3mm (0.25 in.).

2. It requires multiple positions around the circumference of a hole for crack detection.

3. Larger probes prohibit testing between fasteners separated by a distance less than the probe diameter.

For application on an aluminum structure 15 mm (0.6 in.) thick containing steel fasteners, a 100 Hz encircling probe (Fig. 31d) has been developed.

The alternating current coils produce an electromagnetic field that penetrates both the aluminum and steel fastener.

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In addition, the steel fastener acts as a core and tends to concentrate the electromagnetic field. Cracks 13 mm (0.5 in.) long were detected through outer layers of aluminum 8.9 mm (0.35 in.) thick and longer cracks may be detected through even thicker outer layers. The encircling probe can be used where the fastener head spacing is 6.3 mm (0.25 in.) or greater. The major disadvantage of the encircling probe is the absolute measurement system.

Changes in fastener length, fastener permeability, aluminum thickness and conductivity may cause signal variations that interfere with the test. Crack detection sensitivity for laboratory conditions (Fig. 32) has been reported.20 The actual crack detection capability in the field is reduced by signal noise from temperature variations, by separations between the layers and by variation in probe placement. Field experience with a 500 Hz probe confirms that cracks can be reliably detected — 13 mm (0.5 in.) long cracks under a 5 mm (0.2 in.) thick upper layer and 6.3 mm (0.25 in.) long cracks under a 2.5mm (0.10 in.) thick upper layer. Hundreds of test procedures have been developed for crack detection in various aircraft components and structures.

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FIGURE 32. Instrument response at 500 Hz related to crack length and upper layer thickness in typical aircraft structure.

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â– Aircraft Window Belt Splice Cracks Cracks were detected in the first row of rivets above the longitudinal belt splices of aircraft windows. The cracks initiated at fastener holes in the internal (second layer) skin and grew in a longitudinal direction (Fig. 33).

Using a meter or screen display instrument, the test may be performed by scanning between fasteners with a 10 mm (0.4 in.) probe at 2 kHz (Fig. 34).

The crack response is shown in Fig. 34c. To speed up the test, a procedure was developed for scanning in line with and across the aluminum fasteners by using a 15 mm (0.6 in.) diameter probe operating at 1 kHz (Fig. 35). The crack response is shown in Fig. 35c. Both test techniques produce easily interpreted crack responses on a screen display.

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FIGURE 33. Cracks in second layer initiating at aluminum fastener holes: (a) outboard side; (b) inboard side.

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FIGURE 33. Cracks in second layer initiating at aluminum fastener holes: (a) outboard side; (b) inboard side.

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FIGURE 34. Detection of crack in second layer by scanning between fasteners with 10 mm (0.4 in.) probe at 2 kHz: (a) scanning procedure; (b) no crack response; (c) crack response.

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FIGURE 34. Detection of crack in second layer by scanning between fasteners with 10 mm (0.4 in.) probe at 2 kHz: (a) scanning procedure; (b) no crack response; (c) crack response.

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FIGURE 35. Detection of crack in second layer by scanning over fasteners with a 15 mm (0.6 in.) probe at 1 kHz: (a) scanning procedure; (b) no crack response; (c) crack response.

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FIGURE 35. Detection of crack in second layer by scanning over fasteners with a 15 mm (0.6 in.) probe at 1 kHz: (a) scanning procedure; (b) no crack response; (c) crack response.

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Fatigue cracks caused by pressurization cycles were detected in the window belt panels on an airplane with many hours in service (Fig. 36). The window belt was removed from the aircraft and sent to the manufacturer for evaluation.

A section of the cracked panel was used to develop a nondestructive test.

The low frequency eddy current test was performed at a frequency range of 500 Hz to 1 kHz using a 13 mm (0.5 in.) diameter probe. A reference standard was made, simulating the window belt splice joint. It contained a 25 mm (1.0 in.) electric discharge machined notch at the change of thickness area in the window belt panel.

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A nonmetallic straight edge was used to guide the center of the probe directly over the center of the splice. For instruments equipped with meters, the liftoff and phase angles were set so that liftoff drove the meter up scale and passing the probe over the electric discharge machined notch drove the meter down scale. For instruments equipped with oscilloscopes, the phase rotation was null balanced to produce a crack response like that in Fig. 34c.

When a crack signal is obtained, using the low frequency eddy current procedure on the outside of the aircraft, verification requires removal of the interior seats, lining and insulation to conduct a visual or high frequency eddy current check of the internal surface of the window belt panel.

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FIGURE 36. Window belt crack location: (a) area; (b) close view.

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FIGURE 36. Window belt crack location: (a) area; (b) close view.

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18.6.2

Automated Fastener Hole Testing

Inservice test techniques using a driver and receiver sliding probe eddy current system have been developed for improved crack detection in aging aircraft fastener hole tests.13,21 The basic idea of this test system is to slide the probe along a line of fasteners without removing them and to observe the screen display pattern for crack responses. The technique has used a circular probe centered over the installed fastener and the screen display is observed for a crack response. Unfortunately, the results with circular probes are affected by fastener permeability, fastener length, fastener spacing, ambient temperature and probe centering. Flat surface low frequency eddy current probes offset some of these problems. Sliding probe systems, however, offer better crack sensitivity and reliability as well as a wide requency range and good penetration.

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Scanning is done in the direction of crack extension (but it will detect cracks within a 90 degrees angle or less). The crack direction is normally known by the direction of the stress in a given test object. The probes are reflection type and must be used with appropriate instruments. When an aluminum specimen with subsurface cracks is scanned, the fastener response is almost parallel to the liftoff and the crack response has an increasing clockwise phase angle response.

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18.6.3

Conclusion

Eddy current testing is a primary test technique for detecting cracks and corrosion of aircraft structure and engine hardware during maintenance overhaul. These tests are performed in accordance with procedures developed by the manufacturer. Such tests can be used to detect fatigue cracks resulting from cyclic loading during flight, takeoff or landing. They can also be used to detect stress corrosion cracks to indicate the extent of corrosion damage or to identify portions of aircraft structures damaged by fire.

In this way, it is possible to identify damaged components that need to be replaced to restore the craft to airworthiness. Fracture mechanics and nondestructive testing programs have consistently shown that eddy current tests are very reliable in detecting cracks that would be missed by radiography or that require careful cleaning or coating removal to be detected by liquid penetrant or magnetic particle testing.

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In addition, these studies also reveal that very small cracks can be detected by high frequency eddy current tests. As a consequence, aircraft and engine inspections must use eddy current testing to ensure product reliability.

Fortunately, this demand for eddy current testing has generated considerable research and development by aircraft manufacturers, airlines, military and eddy current equipment manufacturers. Development has resulted in advances in instruments, probes and other aspects of the technology.

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18.7

PART 7. Eddy Current Testing of Jet Engines

Eddy current testing is a well established technique for the detection of surface discontinuities in conductive test parts. The United States Air Force and other organizations have used automated eddy current testing to ensure that critical engine components are free of rejectable surface discontinuities.

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18.7.1

Eddy Current Techniques for Jet Engines

Eddy current tests of military aircraft engines can be categorized as manufacturing, depot or field tests. When aircraft engine components are initially produced, eddy current tests of the manufactured parts are often required by specification to ensure that the new engine components are free of undesirable material properties or machining discontinuities. Inservice parts similarly require depot tests at predetermined intervals to ensure that parts are free of small discontinuities in critical, predetermined zones.

Manufacturing tests and depot tests are performed as preventive screening.

Structural integrity requirements are based on fracture mechanics analysis using mission critical parameters for each engine component. For a helicopter engine, conservative service life limits and critical discontinuity sizes are determined by using minimum material properties and other worst case operating conditions. Aerodynamic, heat transfer, stress and life analysis models are used along with the criticality of a failure to determine test zones and discontinuity requirements for critical engine components.

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Jet Engine

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Jet Engine

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The aviation community recognizes that eddy current tests can reliably detect small discontinuities and locate critical discontinuities. Eddy current tests are typically required for discontinuities smaller than 1.8 mm (0.07 in.) long whereas fluorescent liquid penetrant testing is often sufficient for discontinuities over 1.8 mm (0.07 in.) long. Although eddy current requirements have been as small as 0.13 mm (0.005 in.) deep by 0.25 mm (0.010 in.) long, surface requirements are typically in the range of 0.25 × 0.50 mm to 0.75 × 1.50 mm (0.010 × 0.020 in. to 0.030 × 0.060 in.). The required discontinuity sizes, orientations, test areas and surface conditions dictate the test technique. Parameters such as coil type, test frequency, scanning, indexing and filtering are chosen to optimize the ratio of signal to noise for each test.

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18.7.2

Probability of Detection

To demonstrate a technique’s reliability after it is developed, data for probability of detection are collected from fatigue crack specimens of the same material and by using the same parameters. The term probability of detection refers to the statistical protocols used to quantify the effects of test variables on test reliability, specifically on the identification of rejectable discontinuities. Probability of detection data analysis is used to quantify a system’s response and to determine the required test thresholds. Many aircraft manufacturers require potential eddy current test service agencies to submit a test matrix to assess potential test variables and their impact on test sensitivity. Such variables include repeat runs, different operators, different machines and different serial number probes of the same probe type.

Variability test matrices for an automated eddy current test station (for retirement for cause) have identified probe variability to be the most significant cause of variation in the probability of detection data for the test station.

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Therefore, a minimum of two or three probes with the same specifications are used to collect the probability of detection data. Although a greater number of probes would be desirable, cost and schedule constraints typically prohibit additional collections. After the probability of detection data are collected, a statistical fit is performed by using the broadest analysis range that is statistically acceptable. The analysis range must contain all required crack lengths for a given combination of technique and test object. Slope, intercept and scatter parameters are then obtained from the data analysis. These parameters are used to calculate required limits and thresholds necessary to detect required crack sizes with, for example, 90 percent probability at a 95 percent mean confidence level. This requirement is known as the 90/95 requirement.

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Sizing information can also be obtained from the probability of detection results to convert recorded signal amplitude into estimated crack size. It is important to note that estimated crack sizes are inaccurate outside of the probability of detection data analysis angel because of instrument saturation, signal roll-off (where discontinuity sizes exceed settings for signal maxima) and other nonlinearities (Figs. 37 and 38). Therefore, it is crucial that all required discontinuity sizes are sufficiently bounded by the probability of detection data analysis range and that no extrapolations are made for required discontinuity sizes. Although indications with amplitudes outside the probability of detection analysis range will not be accurately sized, they will still be detected and the tested region will receive a reject status.

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FIGURE 37. Probability of detection data for three similar split D shaped (Figs.

40 to 42a) reflection probes on bolt hole.

Legend = probe 1 = probe 2 = probe 3

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FIGURE 38. Typical probability of detection response for three similar wide field probes on flat plate specimens.

Legend = probe 1 = probe 2 = probe 3

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18.7.3

Standard versus Complex Geometries

Within the manufacturing and depot categories, eddy current tests can be further subdivided into standard and complex geometry categories. Standard tests typically include part configurations that lack significant geometry signals or other significant noise. Examples of standard geometries include continuous surfaces, bolt holes and scallops (Fig. 39). Although surface tests are typically free of strong geometry signals, test zones can sometimes run close enough to bolt holes or other features to pick up a geometry signal.

Surface tests can also require thresholds so small that additional work is needed to differentiate discontinuity responses.

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FIGURE 39. Discontinuity locations for fighter engines: (a) corner of cut for balance or drain slot; (b) corner of balance flange scallop; (c) disk rim; (d) air holes for oil drain, fastener or cooling; (e) corner of anti rotation window; (f) through knife edge seal; (g) blade interior.

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FIGURE 39. Discontinuity locations for fighter engines: (a) corner of cut for balance or drain slot; (b) corner of balance flange scallop; (c) disk rim; (d) air holes for oil drain, fastener or cooling; (e) corner of anti rotation window; (f) through knife edge seal; (g) blade interior.

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FIGURE 39. Discontinuity locations for fighter engines: (a) corner of cut for balance or drain slot; (b) corner of balance flange scallop; (c) disk rim; (d) air holes for oil drain, fastener or cooling; (e) corner of anti rotation window; (f) through knife edge seal; (g) blade interior.

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FIGURE 39. Discontinuity locations for fighter engines: (a) corner of cut for balance or drain slot; (b) corner of balance flange scallop; (c) disk rim; (d) air holes for oil drain, fastener or cooling; (e) corner of anti rotation window; (f) through knife edge seal; (g) blade interior.

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Rocket Engine

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Although bolt hole and scallop geometries have significant edge geometries, coil configuration and scan techniques can typically be selected so no appreciable edge signal is present during the tests. However, significant geometry signals can occur if a bolt hole probe is not accurately centered on a bolt hole or precise following of scallops cannot be accomplished. These geometry signals can render the test of such features impractical. Lack of sophisticated probe positioning apparatus can permit unwanted geometry signals. Bolt hole and scallop geometries with very low test requirements force standard geometries to use complex test techniques in order to extract very small discontinuity signals out of the background noise. Complex geometries include broach or dovetail slots, oil drain holes, shaped holes, anti-rotation windows, knife edges, air foils and other geometries that produce large unwanted edge signals (Fig. 39).

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These geometries require special techniques, sophisticated signal processing or both to suppress the large unwanted geometry response. The development of tests for complex geometries typically requires extremely accurate probe centering to obtain a very consistent edge response combined with a signal removal or differencing technique. These edge responses can easily be one hundred times greater than the desired discontinuity response. Often, differencing techniques alone are insufficient for suppressing strong geometry signals and signal processing techniques are required.

Keywords: These edge responses can easily be one hundred times greater than the desired discontinuity response.

Differencing techniques alone are insufficient for suppressing strong geometry signals and signal processing techniques are required.

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â– Signal Processing Signal processing techniques include parametric modeling of edge responses, pattern recognition, frequency discrimination, digital filtering and phase discrimination.

1. Parametric modeling involves adjusting slope, amplitude, direct current offset, phase or other parameters to obtain a best fit to some predetermined model to significantly reduce edge variations.

2. Imaging and pattern recognition techniques can also be used to reduce edge variations.

3. Frequency discrimination techniques can be as simple as selecting the proper filter settings to eliminate unwanted noise but available instrument filter settings are often insufficient to discriminate between an indication and noise.

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4. Digital filters are used to supplement instrument filters and improve the ratio of signal to noise.

5. Phase discrimination techniques can be used to extract discontinuity signals if there is enough phase separation between the discontinuity signal and unwanted signals. Regardless of the signal processing technique applied to a given test, it is important that test parameters are equivalent to those used to collect the probability of detection data.

Without equivalent test parameters, it is difficult to ensure that test thresholds and discontinuity sizing are accurate.

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18.7.4

Eddy Current Probes for Jet Engines

Because probe and coil variations were identified as a significant variable in the automated eddy current test station probability of detection data, great care is taken to control slight variations in the probes. Probes have been studied to assess their parameters, optimize their design and minimize variations from probe to probe.

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■ Split Core Coils Split core coils use two separate receiving coils wound in opposite directions on a split cylinder of ferrite whose cross section resembles two D shapes, one D being the mirror image of the other. The two Ds are back to back so that their cross section looks like a bifurcated circle. These D shaped receive coils are typically bound together with a drive coil wound around them both. For the study of these coils, probe footprints were collected by indexing a probe across a small cross section of a 150 μm (0.006 in.) gage wire in a nonconductive plate.

These footprints illustrate some probe parameters such as coil orientation, coil size and coil tilt (Figs. 40 and 41). With careful alignment, coil positioning can also be easily visualized and quantified. In addition to orientation, size and tilt, the footprint assessment can also provide insight into a probe’s working sensitivity. Results have shown a correlation between a reduced probe footprint and a reduction in the probe’s response to larger fatigue cracks.

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FIGURE 40. Axial alignment of probe having a 4 to 5 mV, split D shaped coil: (a) tilt perpendicular to split; (b) acceptable alignment; (c) tilt in split axis. (a)

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(b)

(c)

FIGURE 41. Footprints of eddy current probes having split D shaped coil: (a) first probe; (b) second probe; (c) third probe.

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FIGURE 41. Footprints of eddy current probes having split D shaped coil: (a) first probe; (b) second probe; (c) third probe.

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FIGURE 41. Footprints of eddy current probes having split D shaped coil: (a) first probe; (b) second probe; (c) third probe.

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Many standard techniques for aircraft engines use a split D shaped coil with the coil axis running normal to the test object (Fig. 42a). A differential coil is used to exclude environmental noise such as temperature variations and bulk material properties. To further increase the coil sensitivity, a separate driving coil is often wound around the two receiving coils. In this configuration, the probe is classified as a differential reflection probe. Probes are generally specified according to coil diameter. One of the D shaped coils has a 0.51 mm (0.020 in.) diameter core. Coils with 1.27 mm (0.050 in.) and 1.52 mm (0.060 in.) diameter cores are some of the larger D shaped coils used for the detection of relatively small discontinuities. The smaller diameter coils provide a greater ratio of signal to noise but require a greater number of indexes to adequately cover the same amount of test area. Several techniques have been used to increase test throughput.

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FIGURE 42. Eddy current probes: (a) split, D shaped reflection coil; (b) wide field bobbin coils; (c) wide field flex coil.

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FIGURE 42. Eddy current probes: (a) split, D shaped reflection coil; (b) wide field bobbin coils; (c) wide field flex coil.

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FIGURE 42. Eddy current probes: (a) split, D shaped reflection coil; (b) wide field bobbin coils; (c) wide field flex coil.

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â– Wide Field Probes The term wide field probes typically refers to probes with circumferentially wound coils whose axes are parallel to the test surface to achieve a wider field of view. Two receiving coils are abutted (so that a circular end of one cylindrical coil is next to the end of the other) such that their currents flow in opposing directions. These two receiving coils are often supplemented with a separate drive coil in the same probe. When the probe is applied with the axis parallel to the test surface, the discontinuity frequency no longer depends on the coil diameter and the coil can be larger so that the field of view is as wide as the contact area between the coil and the test surface. Because part noise is integrated over the contact area of the coil, the practical size of the wide field probe is determined by the required discontinuity detection size and the anticipated noise of typical test surfaces.

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Wide field probes can be further categorized as bobbin coils, shaped coils and flex coils.

1. Bobbin coils generally refer to coils whose circumferential windings are circular or nearly circular (Fig. 42b). Bobbin coils have cores of air, ferrite, annealed steel or other materials.

2. Shaped coils are a slight derivative from the bobbin coils in that the coils are shaped to fit a distinct geometry.

3. Flex coils differ from bobbin and shaped coils in that each coil is housed in a flexible protective jacket that lets the coil be pressed into a geometry to conform to the surface (Fig. 42c).

The main advantage of the wide field probe is the increased throughput provided by a single-pass test. However, the increased throughput comes at a cost.

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The disadvantages of the wide field coils include: (1) a greatly diminished signal response at a non-optimal coil orientation, (2) greater rolloff (where a large discontinuity signal amplitude exceeds the threshold of rejectable indications) and (3) an inability to identify the circumferential location of an indication about a coil.

1. Because the required discontinuity orientation is typically ideal for the wide field coil technique, orientation sensitivity is generally not an issue.

2. Although rolloff at larger discontinuity sizes is of greater concern than rolloff of the D shaped reflection probe, the amplitudes of larger discontinuity sizes are generally much greater than the threshold required for the test. Although discontinuities beyond the rolloff range will be reported at a reduced size, they will still exceed the rejectability threshold and will need to be addressed before acceptance of a part. A typical response related to rolloff is illustrated in Fig. 38 for a wide field probe.

3. Although knowing the circumferential location of an indication can be useful in locating a discontinuity, the axial location of the discontinuity is usually sufficient in approximating and locating a discontinuity. In applications where knowing the circumferential position of a discontinuity is critical, an overlapping test with a coil can be used to help determine the approximate circumferential location of the discontinuity.

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â– Array Probes A second approach to improving test throughput involves an array of eddy current coils. Increasing throughput comes at a cost. Eddy current arrays require more channels. Although multiplexing can be performed to significantly reduce the costs associated with parallel processing of multiple signals, the costs will still be greater than that of a system with a single probe. In addition to the increased equipment costs, sensor and probe costs typically increase as more sensors are added.

Although printed circuit board eddy current coil arrays can be used to reduce sensor costs and provide a flexible coil array, these arrays typically come with a decrease in coil sensitivity because of the single-turn nature of these coils.

In addition to the sensor production costs, there are also costs to normalize coils and ensure their proper sensitivity. After these costs and problems are addressed, the eddy current array approach can offer advantages over single-sensor testing.

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18.7.5

Conclusion

The testing of critical components of aircraft engines requires careful attention to ensure that test parameters are carefully set to meet test requirements.

Automated equipment is essential to precisely position and scan sensors on geometries where slight inaccuracies in coil positioning can produce indications that lead to rejection of an acceptable part. To further improve ratios of signal to noise with very stringent discontinuity requirements, sophisticated signal processing techniques are often applied. Great care is also taken to minimize system variations. Although certain techniques are used to increase test throughput, the driving factors for scheduled tests of critical engine component tests are adherence to the discontinuity requirements and test zones specified by the manufacturer.

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Vertical Liftoff

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Vertical Liftoff

Charlie Chong/ Fion Zhang

http://aircraft.wikia.com/wiki/Bell-Boeing_V-22_Osprey

Vertical Liftoff

Charlie Chong/ Fion Zhang

http://aircraft.wikia.com/wiki/Bell-Boeing_V-22_Osprey

Vertical Liftoff

Charlie Chong/ Fion Zhang

http://aircraft.wikia.com/wiki/Bell-Boeing_V-22_Osprey

Harrier Jump Jet

Charlie Chong/ Fion Zhang

Harrier Jump Jet

Charlie Chong/ Fion Zhang

Harrier Jump Jet

Charlie Chong/ Fion Zhang

https://www.youtube.com/embed/K2Lg-x1tAyE

Harrier Jump Jet

Charlie Chong/ Fion Zhang

Harrier Jump Jet

Charlie Chong/ Fion Zhang

https://www.youtube.com/watch?v=RnYcPJTtV1A

Harrier Jump Jet

Charlie Chong/ Fion Zhang

https://www.youtube.com/watch?v=5TmzW05Qe3A

Che Guevara

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Che Guevara

Charlie Chong/ Fion Zhang

http://en.wikipedia.org/wiki/Che_Guevara

Good Luck!

Charlie Chong/ Fion Zhang